The present invention relates to a method of controlling a granulating apparatus of the type which produces granules or granular material of a desired particle size by adding a binder liquid to powder material which is high in moisture absorption characteristics while fluidizing the powder material.
In the case of known fluidized bed granulating apparatus of the type in which starting powder material is fluidized in a stream of hot air and, while adding a binder liquid to the fluidized powder material, the particles of powder are adhered and agglomerated thus producing granules, the moisture content of the powder material capable of maintaining the proper fluid state has a given range depending on the properties of the powder material so that if the moisture exceeds the upper limit, the flowing movement is rapidly slowed down and lumps are produced. If the moisture content increases further, the flowing movement is stopped so that the powder material solidifies to agglomerate form and the granulating apparatus is rendered inoperable.
With this type of conventional granulating apparatus, it has been the usual practice to use a control method of for example detecting the temperature or humidity of the exhaust air and thereby controlling the temperature of the hot air or the feed rate of the binder. Also, in the case of the batch-type apparatus, it has been usual to use a method of setting a feed time of the binder by a timer while controlling the temperature and flow rate of the hot air constant. On the other hand, with the granulating operation of moisture-absorbing powder material, the required range of moisture contents for maintaining the proper flowing movement has been extremely small so that as for example, the upper limit is about 3% D.B. in the case of amino acid-containing flavoring material and about 5% D.B. in the case of powder soy and exceeding such upper limit of the moisture content has the effect of rapidly deteriorating the fluid state and stopping the flowing movement in a very short period of time.
With these conventional methods of the granulating apparatus, however, it has been impossible to detect such a small change as a change in the exhaust air temperature or humidity and it has been impossible to maintain constant the amount of hot air supply due to for example the clogging of the bag filter incorporated in the apparatus proper in the case of the batch process. Thus, it has been necessary to operate the apparatus with a considerably low moisture content in order to avoid the danger of stopping the flowing movement of the powder material within the apparatus and agglomerating the powder material due to the operations of the apparatus, such as, the repetition of the addition of the binder liquid and the intermediate drying.
Thus, the conventional methods have been disadvantageous in that since it is impossible to detect an increase in the moisture content which would deteriorate the flowing movement thus making it necessary to always monitor the fluid state by visual inspection and also the apparatus must be operated with a relatively low moisture content with the resulting decrease in the growth rate of particles, thus requiring a longer granulating time and also making it difficult to increase the product yield with respect to the desired particle size.